This invention relates to a method and device designed to drill countersunk holes automatically to extremely fine tolerances in elements which may be in any position, and in particular in elements whose position cannot be exactly ascertained.
For example, the method in accordance with the invention can be usefully applied in the aeronautical industry, in riveting operations used to fix panels to the ribs which constitute the aircraft structure.
The following description relates particularly to this specific case, although the invention is not limited to the aeronautical industry but could also be used to advantage in other industries such as the automobile industry.
In aircraft contruction, the sheet metal used to make the fuselage and wings is secured to the ribs of the structure by rivets inserted into holes drilled by automatic machine tools.
The various parts must be machined to very fine tolerances; rivets, in particular, must be exactly flush with the outer surface of the wing to reduce drag.
For this reason each hole must be drilled with the highest precision, with tolerances of approx. 20-30 microns in the countersinking depth.
Although current automatic machine tools are extremely precise, this particular type of job presents great difficulties because the holes must be drilled in curved surfaces, and it is a very difficult, laborious job to determine the position with the necessary precision.
Controlling the movements of these machines via software therefore requires the use of complex algorithms, although there is no guarantee that the result will be sufficiently precise, as this also depends on exact pre-positioning of the part to be drilled, which is always difficult to obtain. With known systems the sheet metal to be drilled is placed against the ribs and fixed at a reasonable number of points; the necessary holes are drilled, following which the structure must be dismantled in order to deburr around each hole before riveting and final fixing.
As mentioned, during this job exact positioning of the workpiece is essential, and such positioning is very laborious and extremely difficult to obtain.
There is consequently a felt need in the industry for equipment which enables these holes to be drilled to the required tolerances even when the plate to be drilled is not positioned with the necessary precision.